1. Field of the Invention
The present invention relates generally to printing presses and more particularly to a method and device for providing a printing plate to a plate cylinder during an automatic plate change operation.
2. Background Information
U.S. Pat. No. 5,390,603 discloses a method for automatic changing of printing plates on a plate cylinder of a printing press. The plate cylinder has a locking or clamping device for a lead edge and trail edge of the printing plate. To change the printing plates, the plate cylinders are driven to a release position and the trail edge of the plates is released. The plate cylinders are then turned in the opposite direction to unwind the printing plates. A new plate for each cylinder is then fed in by suction arms which hold the plate so that the lead edge of the new plate can be attached to the lead edge locking device.
U.S. Pat. Nos. 5,495,805 and 5,443,006 disclose devices for carrying away and supplying printing plates to a plate cylinder. In these patents, a magazine has two holding shafts into which cassettes are insertable from above. One cassette is originally empty and can receive used printing plates, while the other holds new printing plates to be attached to the plate cylinder. A plate supplying apparatus takes plates from the new plate cassette and a plate removing apparatus removes the plates from the plate cylinder and places them in the old plate cassette. The plate removing apparatus includes suction elements disposed in a first carriage movable by a driving apparatus along a first guide. To remove the plate, the rear-edge clamping device of the plate cylinder is released, so that the inherent elasticity of the printing plate causes the rear edge of the printing plate to bump against a guide roller. The plate cylinder is then rotated backwardly so that the plate is inserted into the magazine, where the suction elements are activated and, simultaneously with a further backward rotation of the plate cylinder, the first carriage with the suction elements moves the old printing plate into an upper position, the lead edge having also been released. The old plate is then placed in the old plate cassette. To supply a new plate, the plate supplying apparatus has suction elements on a second carriage. The lead edge of the new printing plate is carried to the front-edge clamping device of the plate cylinder. The plate is aligned and clamped. The plate cylinder is rotated in the forward direction, pulling the printing plate onto its outer cylindrical surface. Due to a weakened vacuum of the suction elements, the printing plate still can be held by the suction elements while sliding over the suction elements sue to the plate cylinder rotation. Before the printing plate trail edge passes the last two suction elements, the suction elements are deactivated and a pressing-in roller presses the trail edge radially into the trail-edge clamping device. The trail edge is clamped and the plate tensioned.
U.S. Pat. No. 5,701,822 discloses a device for changing printing plates on an offset printing press. The plate cylinder has an automatic clamping device in a gap or channel of the plate cylinder. A holding element to aid in position of the printing plate on the plate cylinder is provided, as is a printing plate loading unit, which holds a printing plate to be loaded. The holding element grips a plate to be inserted and positions the lead edge against the plate cylinder. As the plate cylinder rotates, the lead edge slips into the gap or channel and is clamped by the automatic clamping device. The printing form then can be tautly wrapped around the circumference of the plate cylinder and the trail edge fastened therein, aided by a roller.
A problem with the devices and methods described above is that the lead edge of the plate may skew or become misaligned with the plate gap before insertion. This skew or misalignment can cause malfunction of an automatic plate device. Especially when used with very small gap plate cylinders, the above methods and devices are prone to malfunction or cannot be used at all.
U.S. Pat. Nos. 5,553,544 and 5,284,093 show small gap plate cylinders, where the trail and lead edges of the plate are inserted into the same gap so that only a small space separates them. U.S. Pat. No. 5,678,487 also shows a small gap plate cylinder and discloses a tucking device for the trail edge. These three patents, as well as U.S. Pat. Nos. 5,495,805 and 5,443,006 discussed above, are hereby incorporated by reference herein.
An object of the present invention is to provide an improved method for automatically attaching printing plates to a plate cylinder. An additional object is to provide an improved device for providing a printing plate to the plate cylinder.
The present invention provides a method for automatically providing a printing plate to a plate cylinder including the steps of aligning a first edge of a printing plate against a cylinder adjacent to a plate cylinder, moving the first edge of the printing plate into a gap in the plate cylinder, and wrapping the printing plate about the plate cylinder.
The present method provides for better alignment of the plate before insertion into a gap in the plate cylinder. The lead edge of the plate thus more reliably fits into the gap in proper alignment. Thus with even small gapped plate cylinders, the plate can be inserted more reliably.
The present method further may include the step of fastening the first edge in the gap. The plate cylinder may then be rotated to wrap the plate about the plate cylinder. A tucking device may then be used to aid in inserting the trail edge of the plate into the gap.
Preferably, the adjacent cylinder is a blanket cylinder of an offset printing press. The blanket cylinder advantageously provides a large adjacent surface on which to align the plate.
The present invention also provides a method for attaching a printing plate to a plate cylinder comprising the steps of holding a printing plate through a first set of grippers, moving the first set of grippers so that a lead edge of the plate aligns against a cylinder adjacent to the plate cylinder, holding the printing plate through a second set of grippers, releasing the fist set of grippers, moving the second set of grippers toward a gap in the plate cylinder, and inserting the lead edge of the printing plate into the gap.
The present invention also provides a device for attaching a printing plate to a plate cylinder having a first set of grippers for gripping a printing plate and aligning the printing plate against a cylinder adjacent to the plate cylinder and a second set of grippers for moving the printing plate toward a gap in the plate cylinder.
Advantageously, the first set of grippers includes spring-mounted grippers, so that the plate may align against the adjacent cylinder. The first set of grippers preferably moves in a direction solely radial to the adjacent cylinder. The second set of grippers preferably is movable in a direction substantially radial to the plate cylinder.
The device may further include a tucking device for tucking in the trail edge of the printing plate.
The present invention also provides an offset printing press having a plate cylinder with a plate locking mechanism and having a receiving gap on an outer surface, a blanket cylinder forming a nip with the plate cylinder; and a plate attachment device having a first set of grippers for gripping a printing plate and aligning the printing plate against the blanket cylinder and a second set of grippers for moving the printing plate toward the receiving gap.